1. Field of the Invention
The present invention relates to a deprotection method for removing a protecting group from a polymer obtained by polymerization while protecting a functional group, or a polymer obtained by modification of a side chain. In particular, the invention relates to a method of producing a polymer to be used as a chemically amplified photoresist material.
2. Description of the Related Art
Along with an increase in the integration density of integrated circuits, formation of more minute patterns has been required in recent years. In processing into patterns having a size of 0.2 μm or less, chemically amplified resist using an acid as a catalyst has mainly been used. As an exposure light source upon this processing, high energy radiation such as KrF excimer laser light, ArF excimer laser light, EUV, or electron beams has been used. In particular, the electron beam lithography used as a microfabrication technology has been inevitable as a processing method of a photomask blank to be used for the formation of a photomask for manufacturing a semiconductor.
In general, a resin comprised by a resist composition and designed for a KrF excimer laser light or electron beam as high energy radiation for pattern exposure comprises a unit structure having a phenolic hydroxyl group as a functional group giving good substrate adhesion, while development of resist for EUV exposure has also been promoted in this direction. As a typical example of the unit structure having a phenolic hydroxyl group, 4-hydroxystyrene is well-known. A hydroxystyrene monomer does not have high stability so that a polymer having a hydroxystyrene unit is often obtained by polymerizing or copolymerizing an acetoxystyrene monomer having good stability and polymerizability and then deacetylating the resulting polyacetoxystyrene derivative by using methanolysis with triethylamine/methanol or by using a base such as ammonia water, sodium hydroxide, sodium methoxide or hydroxylamine hydrochloride.
JP 01-188502A/1989 discloses a method of carrying out a deprotection reaction of a polymer having an acetoxystyrene unit structure and being a typical resist material, in the suspension as an aqueous reaction mixture. JP 01-188502A/1989 discloses many usable bases. These bases may be usable even in a homogeneous deprotection reaction conducted by dissolving both a polymer protected with an acyl group, which is a reaction substrate, and a deprotecting reagent in an organic solvent.
Although the polymer having a hydroxystyrene unit structure to be used as a base polymer for the resist includes a homopolymer of hydroxystyrene, a functional structure may be introduced into the polymer in the form of an ester bond as a controlling factor of the physical properties or functions of the resulting resist. In order to synthesize such a polymer, it is the common practice to obtain a targeted polymer by copolymerizing (meth)acrylic acid ester derived from an aliphatic alcohol having the above-described functional structure (for example, JP 2002-62652) or vinyl aromatic carboxylic acid ester (for example, JP 2007-254495) with acetoxystyrene or a vinyl aromatic compound having a phenolic hydroxyl group protected with an acetyl group and then deprotecting the resulting polymer. The term “(meth)acrylic acid” means methacrylic acid and/or acrylic acid.
In deprotection of such a polymer comprising an ester derived from an aliphatic alcohol as described above, selective cleavage of a phenol ester is required while maintaining the ester derived from an aliphatic alcohol. Accordingly, hydrolysis with a weak base has conventionally been conducted. As a stable industrial production process, it is difficult to adopt a reaction in use of ammonia water as a weak base, because volatility of ammonia prevents the reaction temperature from being higher. A methanolysis deprotection reaction in methanol in use of triethylamine as a base is therefore ordinarily used. The triethylamine is an organic base.